Methods of manufacturing tubes and/or rods of glass are well known. For example, one method of forming a glass tube or rod is described in U.S. Pat. No. US2009793. In this so-called Vello process, molten glass flows from a vessel through an opening at the bottom of the vessel as a column of molten glass. A blow tube positioned within the flow at the opening injects a gas such as air through a cylindrical or conical section and into the column to create a hollow centerline in the column, thus forming a tube of molten glass having a hollow interior. The direction of flow of the glass tube is thereafter changed from vertical to a non-vertical orientation, such as horizontal. Downdraw variations of this method are also known wherein the glass tube is drawn vertically downward without changing the direction of flow.
Devitrification occurs when a glass transforms into a stable crystalline form. Above a certain temperature, referred to as the devitrification or liquidus temperature, crystallization is inhibited. The liquidus temperature (or conversely the liquidus viscosity) directly influences whether crystallization is likely to occur for a given process.
For a multi-component glass, different devitrification limits often exist in the glass depending on the relevant crystal phase and location of formation. To determine the tendency for a glass to undergo devitrification one must take into account the devitrification limit of relevance to the particular process. In the case of glasses produced by drawing processes, this is generally the crystallization that occurs at the phase boundaries described above.
The tendency of a glass to crystallize can differ considerably from glass to glass, however, and will vary with the chemical composition of the glass. The tendency to crystallize can be influenced by varying the composition. However, variations in composition can significantly impact other properties of a glass, which are often determined by the intended use of the glass. Therefore, it may not be possible to select a glass composition having the requisite stability with respect to crystallization (i.e. not crystallize) and still meet the predetermined physical characteristics dictated by the intended use with conventional methods and equipment.